Method and system for localizing a medical tool

ABSTRACT

A method of localizing a medical tool, the method comprising: (1) generating an image of a reference target with a camera that is attached to a medical tool, wherein the reference target is remote from the medical tool and located in a room at a known position relative to a coordinate system; and (2) determining the position of the medical tool relative to the coordinate system at least partially on the basis of the generated image of the reference target. Examples of medical tools that can be localized in accordance with the present invention include medical imaging devices, surgical instruments, and bite blocks.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to provisional patent application Ser.No. 60/491,634 entitled “Method and System for Localizing a MedicalImaging Probe” filed Jul. 30, 2003, the entire disclosure of which isincorporated herein by reference.

This application is also a continuation-in-part of pending patentapplication Ser. No. 10/230,986 entitled “Method and Apparatus forSpatial Registration and Mapping of a Biopsy Needle During a TissueBiopsy” filed Aug. 29, 2002, the entire disclosure of which isincorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates generally to the localization of a medicaltools, particularly medical imaging devices. In particular, the presentinvention relates to the localization of a medical imaging probe inreal-time as the probe is used in connection with generating a medicalimage of a patient.

BACKGROUND OF THE INVENTION

With various medical procedures in which medical images of a patient aregenerated using a medical imaging device such as a medical imagingprobe, it is often important to precisely know the position of contentdepicted in a medical image relative to a fixed coordinate system. Thiscontent depicts a region of interest (ROI) of the patient. Such aposition determination allows for precise patient diagnoses, preciseformulation of treatment plans, precise targeting of therapy treatments,and the like.

For example, when preparing an external beam radiation treatment planfor treating prostate cancer, it is highly important to target theradiation beam as closely as possible to diseased regions of theprostate to thereby minimize damage to nearby healthy tissue. In thiscase, the ROI is a diseased region of the prostate or the entireprostate with a minimized treatment margin surrounding the prostate.Once the diseased region is identified in the medical images of theprostate, the question becomes how to accurately target the radiationbeam to the ROI and spare adjacent critical structures. To achieve suchtargeting, it is desirable to spatially register the medical imagesrelative to the fixed coordinate system of the radiation beam source. Inthis process, the known and relatively constant variables are theposition of the radiation beam relative to the fixed coordinate system,the position of the ROI relative to the probe's field of view, and theprobe's field of view relative to the probe's position. The missing linkin this process is the position of the medical imaging probe relative toa coordinate system such as the coordinate system of the radiationsource at the time the probe obtains data from which the medical imageof the patient is generated.

A variety of techniques, referred to generally as localization systems,are known in the art to determine the position of a medical imagingprobe relative to a fixed coordinate system. Examples of knownlocalization systems can be found in U.S. Pat. Nos. 5,383,454,5,411,026, 5,622,187, 5,769,861, 5,851,183, 5,871,445, 5,891,034,6,076,008, 6,236,875, 6,298,262, 6,325,758, 6,374,135, 6,424,856,6,463,319, 6,490,467, and 6,491,699, the disclosures of all of which areincorporated herein by reference.

For example, it is known to mount the medical imaging probe in apositionally-encoded holder assembly, wherein the assembly is located ata known position in the coordinate system (and therefore the probe'sposition in the coordinate system is also known) and wherein the probeis moveable in known increments in the x, y, and/or z directions.However, because such localization systems require the use of a holderassembly, the probe's range and manner of movement is limited to what isallowed by the encoder rather than what is comfortable or most accuratefor the medical professional and patient.

It is also known to mount a medical imaging probe in a holder assembly,wherein light sources such as light emitting diodes (LEDs) are affixedeither to the probe itself or to the holder assembly, and wherein acamera is disposed elsewhere in the treatment room at a known positionsuch that the LEDs are within the camera's field of view. Applyingposition determination algorithms to points in the camera images thatcorrespond to the LEDs, the probe's position relative to the system'sfixed coordinate system can be ascertained.

In connection with freehand medical imaging probes, similar localizationsystems are used wherein LEDs are affixed to the probe, wherein a camerathat is disposed elsewhere in the treatment room at a known location isused to generate images of those LEDs, and wherein a positiondetermination algorithm is used to process the camera images to localizethe probe in 3D space.

However, because treatment rooms typically offer a limited variety ofchoices for camera placement locations, it is often the case that aclose spatial relationship cannot be maintained between the camera andthe LEDs it seeks to track. Thus, it is believed that these knowncamera-based localization systems suffer from potential line-of-sight(LOS) problems as people in the treatment room move about or as theprobe is moved about during the imaging process. These same problems arebelieved to exist in connection with localizing medical tools other thanmedical imaging devices (such as surgical instruments).

SUMMARY OF THE INVENTION

In view of these and other opportunities for improvement in conventionallocalization systems, the inventors herein have developed the presentinvention. As a unique and elegantly simple improvement to the prior artdiscussed above, the inventors herein have, in their preferredembodiment, attached a tracking camera to a medical imaging probe andplaced the reference target tracked by the camera elsewhere in thetreatment room at a known location. Because there are a much greaternumber of options for reference target placement in a treatment roomthan there are for camera placement due to the reference target's smallsize and easy maneuverability, the present invention allows for a closespatial relationship to be maintained between the tracking camera andthe reference target, thereby minimizing the risk for LOS problems.Further, the configuration of the present invention can provide improvedaccuracy at lower cost by avoiding the long distances that are usuallypresent between the LEDs and room-mounted cameras of conventionalsystems.

According to one aspect of the invention, disclosed herein is a methodof localizing a medical tool, the method comprising: (1) generating animage of a reference target with a camera that is attached to a medicaltool, wherein the reference target is remote from the medical tool andlocated in a room at a known position relative to a coordinate system;and (2) determining the position of the medical tool relative to thecoordinate system at least partially on the basis of the generated imageof the reference target.

In preferred embodiments, the medical tool can be a medical imagingdevice (such as a freehand ultrasound probe), a surgical instrument, ora bite block, as described in greater detail below.

Also disclosed herein is a system for localizing a medical imagingdevice, the system comprising: (1) a reference target having a knownposition in a fixed coordinate system; (2) a medical imaging devicehaving a field of view and being configured to receive data from which amedical image of a patient is generated, the medical imaging devicebeing remote from the reference target; (3) a camera attached to themedical imaging device for tracking the reference target and generatingat least one image within which the reference target is depicted; and(4) a computer configured to (a) receive the camera image and (b)process the received camera image to determine the position of themedical imaging device's field of view relative to the coordinatesystem.

Also disclosed herein is a system for localizing a medical tool, thesystem comprising: (1) a medical tool for use in a medical procedurewith a patient; (2) a localization system associated with the medicaltool that locates the medical tool in a three-dimensional coordinatesystem, the localization system comprising a reference target having afixed and known position in the coordinate system, the reference targetbeing remote from the medical tool; and (3) a computer in communicationwith the localization system, the computer being programmed to (a)receive data from the localization system, and (b) determine theposition of the medical tool in the coordinate system at least partiallyon the basis of data received from the localization system.

According to another aspect of the present invention, disclosed hereinis a medical tool having a tracking camera attached thereto in a knownspatial relationship with respect to a point of interest on the medicaltool. The camera is preferably attached to the tool such that the cameraimages a reference target remote from the medical tool while the medicaltool is being used as part of a medical procedure, and wherein thereference target is disposed in the same room as the medical tool at aknown position in the room relative to a 3D coordinate system.

According to yet another aspect of the present invention, disclosedherein is a computer programmed with executable instructions to processcamera images received from a medical tool-mounted tracking cameratogether with known position variables to determine the position of themedical tool relative to the coordinate system.

In a preferred embodiment wherein the medical tool is an imaging probe,the tracking camera is attached to the imaging probe at a known positionand orientation with respect to the imaging probe's field of view.Further, the reference target is located in the treatment room at aknown position in the coordinate system and within the field of view ofthe tracking camera as the probe is put to use. The reference targetincludes a plurality of markings that are identifiable within the cameraimages, wherein the markings have a known spatial relationship with eachother. On the basis of these known variables, a computer programmed witha position determination algorithm can process images from the trackingcamera in which the reference target markings are identifiable todetermine the position of the probe relative to the coordinate system.As a result of determining the probe's positioning relative to thecoordinate system, medical images generated through the use of the probecan be spatially registered to that same coordinate system.

The localization technique of the present invention is suitable for usewith any medical procedure in which spatially registered medical imagesor accurate localized treatments are useful, including but not limitedto the planning and/or targeting of spatially localized therapy (e.g.,spatially localized drug delivery, spatially localized radiotherapyincluding but not limited to external beam radiation therapy treatmentplanning, external beam radiation treatment delivery, brachytherapytreatment planning, brachytherapy treatment delivery, etc.), pre-biopsyplanning, and biopsy execution.

The preferred imaging modality for use with techniques of the presentinvention is ultrasound. However, it should be noted that other imagingmodalities may also be used, including but not limited to imagingmodalities such as x-ray, computed tomography (CT), cone-beam CT, andmagnetic resonance (MR).

These and other features and advantages of the present invention will bein part pointed out and in part apparent upon review of the followingdescription and the attached figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram overview of a preferred embodiment of thelocalization system of the present invention, wherein a transrectalultrasound probe is localized;

FIG. 2 is a block diagram overview of a preferred embodiment wherein thelocalization system uses a transabdominal ultrasound probe;

FIG. 3 is a depiction of the preferred embodiment wherein thelocalization system uses a transabdominal ultrasound probe;

FIG. 4 illustrates a preferred reference target pattern;

FIG. 5 illustrates an exemplary localizable surgical instrument inaccordance with the localization technique of the present invention; and

FIG. 6 illustrates an exemplary localizable bite block in accordancewith the localization technique of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 illustrates an overview of a preferred embodiment of thelocalization system of the present invention, as applied to prostatetreatment via an external beam radiation therapy procedure.Commonly-owned pending application Ser. No. 10/286,368, filed Nov. 1,2002, (the entire disclosure of which is incorporated herein byreference) discloses an exemplary system for external beam radiationtherapy for which the present localization system is well suited. InFIG. 1, a linear accelerator (LINAC) 250 serves as a source of radiationbeam energy for treating prostate lesions. Because of the presentinvention's probe localization, this beam of energy can be preciselytargeted to diseased regions of the prostate 110. However, as notedabove, the localization system is also highly suitable for use withother medical procedures. Further, the target of medical imaging for thepresent invention need not be limited to a patient's prostate. Althoughspatial registration for medical images of a patient's prostaterepresents a unique and highly useful application of the presentinvention given the considerations involved with prostate treatment dueto daily movement of the prostate within the patient, the medicalimaging target of the present invention can be any soft tissue site of apatient's body including but not limited to the pancreas, kidney,bladder, liver, lung, colon, rectum, uterus, breast, head, neck, etc.Most internal organs or soft tissue tumors that move to some degreewithin the patient would be candidates for targeting using thelocalization approach of the present invention.

In FIG. 1, a target volume 110 (or ROI) is located within a workingvolume 102. For the example of FIG. 1, the target volume 110 would be apatient's prostate or a portion thereof, and the working volume 102would be the patient's pelvic area, which includes sensitive tissuessuch as the patient's rectum, urethra, and bladder. Working volume 102is preferably a region somewhat larger than the prostate, centered on anarbitrary point on a known coordinate system 112 where the prostate isexpected to be centered during the external beam radiation therapyprocedure.

A medical imaging device 100, in conjunction with an imaging unit 104,is used to generate medical image data 206 corresponding to objectswithin the device 100's field of view 101. The device 100 may be aphased array of transducers, a scanned transducer, receiver, or anyother type of known medical imaging device, either invasive ornon-invasive. During a planning session or treatment session forexternal beam radiation therapy, the target volume 110 will be withinthe imaging device's field of view 101. Preferably, the medical imagingdevice 100 is an ultrasound probe and the imaging unit 104 is anultrasound imaging unit. Even more preferably, the ultrasound probe 100is a transabdominal or linear array imaging probe, a breast imagingprobe, a transrectal ultrasound probe, or an intracavity ultrasoundprobe. Together, the ultrasound probe 100 and ultrasound imaging unit104 generate a series of spaced two-dimensional images (slices) of thetissue within the probe's field of view 101. Although ultrasound imagingis the preferred imaging modality, as noted above, other forms ofimaging that are registrable to the anatomy may be used in the practiceof the present invention.

Also, in the example of FIG. 1, the imaging probe 100 is a freehandimaging probe. It is believed that the present invention is particularlyvaluable for use in connection with localizing freehand probes because,while freehand probes provide medical practitioners with unparalleledmaneuverability during imaging, they also present difficulties when itcomes to localization because of that maneuverability. However, giventhe present invention's localization abilities, a medical practitioner'sfreedom to maneuver the imaging probe is not hindered by the constraintsinherent to conventional localization techniques. It is worth notingthough, that in addition to localizing freehand probes, the presentinvention can also be used to localize non-freehand probes such asprobes that are disposed in a holder assembly or articulable arm of somekind.

It is important that the exact position and orientation of ultrasoundprobe 100 and its field of view 101 relative to the knownthree-dimensional coordinate system 112 be determined. A preferred pointof reference for the coordinate system, in external beam radiationtherapy applications, is the machine isocenter of the LINAC (LinearAccelerator) 250. This isocenter is the single point in space aboutwhich the LINAC gantry and radiation beam rotates. To localize theultrasound probe to the coordinate system 112, the localizationtechnique of the present invention is used.

This localization technique uses a frameless stereotactic system whereina tracking camera 200 is attached to the ultrasound probe 100 at a knownposition and orientation relative to a point of interest on the probe(preferably, within probe's field of view 101). When it is said that thetracking camera is “attached” to the ultrasound probe, it should beunderstood that this would include disposing the tracking camera on theprobe directly via a single enclosure combining the two, disposing thetracking camera on the probe through a collar around the probe, whereinthe tracking camera is directly affixed to the collar via aclamshell-like device, attaching the camera to the probe directly with aclamp. As would be understood by those of ordinary skill in the art, anyof a number of known techniques can be used to appropriately attach thecamera to the probe. Further still, the tracking camera 200 may also bedetachable from the probe, although this need not be the case. Thepreferred attachment method is to incorporate a single housing thatencompasses the camera 200 (except for the camera lens 252) and theprobe 100 (except for the active transducer coupling window region), asshown in FIG. 3.

Various camera devices may be used in the practice of the presentinvention including but not limited to a CCD imager, a CMOS sensor typecamera, and a non-linear optic device such as a camera having a fish-eyelens (which allows for an adjustment of the camera field of view 201 toaccommodate volumes 102 of various sizes). In general, a negativecorrelation is expected between an increased size of volume 102 and theaccuracy of the spatial registration system. Also, tracking camera 200preferably communicates its image data 204 with computer 205 as per theIEEE-1394 standard.

Camera 200 is preferably mounted at a position and orientation on theprobe 100 that minimizes reference target occlusion caused by theintroduction of foreign objects (for example, the physician's hand,surgical instruments, portions of the patient's anatomy, etc.) in thecamera field of view 201. Further, it is preferred that the camera 200be mounted on the probe 100 as close as possible to the probe's field ofview (while still keeping reference target 202 within camera field ofview 201) because any positional and orientation errors with respect tothe spatial relationship between the camera and probe field of view aremagnified by the distance between the camera and probe field of view. Apreferred location of the camera attachment to the probe matches thelocation of the hand grip for manipulation of the probe. The camera lensviews above the hand grip toward the reference target and the imagingprobe field of view is below the hand grip and probe.

A reference target 202 is disposed at some location, preferably fixedand preferably above or below the patient examination table, in the room120 that is within the camera 200's field of view 201 and known withrespect to the coordinate system 112. Preferably, reference target 202is positioned such that, when the probe's field of view 101 encompassesthe target volume 110, reference target 202 is within camera field ofview 201. For external beam radiation therapy of the abdominal region,one preferred location of the reference target 202 is in the shadow trayor blocking tray of the LINAC. The block tray in some LINACconfigurations inserts into the wedge tray slot. Another preferredlocation is in the wedge tray of the LINAC. The wedge tray in most LINACconfigurations is located immediately on the treatment head of the LINACgantry. The reference target can be placed in the selected tray slot ofthe LINAC and used to localize the targeting system, and then removedfrom the tray just prior to delivering the radiation treatment.

Reference target 202 is preferably a planar surface supported by sometype of floor-mounted, table-mounted, or ceiling-mounted structure.Further, reference target 202 includes a plurality of identifiable marks203 thereon, known as fiducials. Marks 203 are arranged on the referencetarget 202 in a known spatial relationship with each other.

The identifiable marks 203 are preferably passive reflectors or printedmarks visible to the camera 200 such as the intersection of lines on agrid, the black squares of a checkerboard, or some other pattern ofmarkings on the room's wall or ceiling. FIG. 4 depicts a preferredcheckerboard pattern for the reference target 202, wherein some of thecheckerboard marks 203 include further geometric shapes and patterns.

However, other types of fiducials may be used such as light emittingdiodes (LED's) or other emitters of visible or infrared light to whichthe camera 200 is sensitive. Any identifiable marks 203 that aredetectable by the camera 200 may be used provided they are disposed in aknown spatial relationship with each other. Further still, the cameracan be replaced by an electromagnetic sensor or acoustic sensor, and thereference target replaced with electromagnetic emitters or acousticemitters.

It is advantageous for the marks 203 to be arranged in a geometricorientation, such as around the perimeter of a rectangle or thecircumference of a circle. Such an arrangement allows computer software206 to apply known shape-fitting algorithms that filter out erroneouslydetected points to thereby increase the quality of data provided to theposition-determination algorithms. Further, it is preferable to arrangethe marks 203 asymmetrically with respect to each other to therebysimplify the process of identifying specific marks 203. For example, themarks 203 may be unevenly spaced along three sides of a rectangle oralong a circular arc.

The number of marks 203 needed for the reference target is a constraintof the particular position-determination algorithm selected by apractitioner of the present invention. Typically a minimum of threemarks 203 are used. In the preferred embodiment of FIG. 4, acheckerboard pattern with numerous marks 203 is used. In general, thepositional and orientational accuracy of the localization systemincreases as redundant marks 203 are added to the reference target 202.Such redundant marks 203 also help minimize the impact of occlusion. Thesize of the marks 203 is unimportant provided they are of sufficientsize for their position within the camera image to be reliablydetermined.

To calibrate the tracking camera 200 to its surroundings, the camera 200is placed at one or more known positions relative to the coordinatesystem 112. In one preferred embodiment, the known positions of thecamera relative to the target in the coordinate system are determined byprecisioned machined mounting positions of exact known location in ametal plate into which the camera is inserted. When the camera 200 isused to generate an image of the reference target 202 from such knownpositions, the images generated thereby are to be provided to computer205. The positions provide for placing the camera at variousorientations which are communicated to the software. Software 206 thatis executed by computer 205 includes a module programmed with executableinstructions to identify the positions of the marks 203 in the image.The software 206 then applies a position-determination algorithm todetermine the position and orientation of the camera 200 relative to thereference target 202 using, among other things, the known cameracalibration positions, as is known in the art. Once the position andorientation of the camera 200 relative to the reference target 202 areknown from one or more positions and at one or more orientations withinthe coordinate system 112, the computer 205 has calibration data thatallows it to localize the position and orientation of the camera at alater time relative to the coordinate system 112. Such calibration canbe performed regardless of whether the camera 200 is disposed on theprobe 100. It may also be performed with the camera 200 disposed on theprobe 100. The working volume is determined by the size of the region ofthe field of view of the camera relative to the visibility of the activesources or passive targets.

After calibration has been performed, the ultrasound probe 100 (withcamera 200 attached thereto at a known position and orientation relativeto the probe's field of view 101) can be used in “freehand” fashion withits location determined by computer 205 so long as the reference target202 remains in the camera field of view 201. When subsequent cameraimage data 204 is passed to computer 205 via any known connection suchas Firewire (IEEE 1394), Camera Link, or other suitable methods,software 206 (which may be instructions stored in the computer's memory,hard drive, disk drive, on a server accessible by the computer 205, orin other similar manner) applies similar position-determinationalgorithms to determine the position and orientation of the camera 200relative to the reference target 202. By derivation, software 206 isthen able to (1) determine the position and orientation of the camera200 relative to the coordinate system 112 (because the position of thereference target 202 in coordinate system 112 is known), (2) determinethe position and orientation of the probe field of view 110 relative tothe coordinate system 112 (because the position and orientation of thecamera 202 relative to the probe field of view 101 is known and because,as stated, the position and orientation of the camera 200 relative tothe coordinate system 112 has been determined), and (3) determine theposition and orientation of the content of the ultrasound image producedby the ultrasound probe 100 relative to the coordinate system 112(because the ultrasound image contents have a determinable spatialrelationship with each other within the probe's field of view 101 andbecause the relationship between the coordinate system and the cameraare determinable based upon the camera calibration and the knownrelationship between the target and the coordinate system).

Position-determination algorithms are well-known in the art. Examplesare described in Tsai, Roger Y., “An Efficient And Accurate CameraCalibration Technique for 3D Machine Vision”, Proceedings of IEEEConference on Computer Vision and Pattern Recognition, Miami Beach,Fla., 1986, pages 364-74 and Tsai, Roger Y., “A Versatile CameraCalibration Technique for High-Accuracy 3D Machine Vision MetrologyUsing Off-the Shelf TV Cameras and Lenses”, IEEE Journal on Robotics andAutomation, Vol. RA-3, No. 4, August 1987, pages 323-344, the entiredisclosures of which are incorporated herein by reference. A preferredposition-determination algorithm is an edge-detection, sharpening andpattern recognition algorithm that is applied to the camera image tolocate and identify specific marks 203 on the target 202 with subpixelaccuracy. The algorithm uses information from the camera image to locatethe edges or corners of the reference target objects in space relativeto each other and between light and dark areas. Repeated linearminimization is applied to the calculated location of each identifiedmark 203 in camera image coordinates, the known location of eachidentified point in world coordinates, vectors describing the locationand orientation of the camera in world coordinates, and various otherterms representing intrinsic parameters of the camera. The position andorientation of the ultrasound image is computed from the position andorientation of the camera and the known geometry of the probe/camerasystem.

One embodiment of the reference target may include sub-regions withadditional patterns that are different in each sub-region. The softwareuses pattern recognition to analyze the presence and type of eachsub-region pattern to determine which portion of the reference target isbeing viewed by the camera whenever the entire target is not visible tothe camera. This information is used to extend the useful operationalarea or volume for localization of an image or surgical instrument.

Thus, as the ultrasound probe 100 is used to image the target volume 110while the camera 200 tracks the reference target 202, camera image data204 is provided to computer 205 and ultrasound image data 103 isprovided to the ultrasound imaging unit 104 via a connection such as acoaxial cable. Software 206 executed by the computer operates to processthe camera images received from the tracking camera 200 to localize theprobe 100 through the above-described position determination algorithm.Once the probe 100 has been localized, the computer can also spatiallyregister the ultrasound images 208 received via a connection such as adigital interface like Firewire or analog video from the ultrasoundimager unit 104 through image registration techniques known in the art.This process is capable of occurring in real-time as the ultrasoundsound probe is used to continuously generate ultrasound image data.

As mentioned above, in addition to the localization of medical imagingdevices, the techniques of the present invention can also be applied tothe localization of medical tools such as surgical instruments, biteblocks, and the like.

FIG. 5 depicts an example of a localizable surgical instrument 500. InFIG. 5, the surgical instrument 500, which may be a biopsy needle, aneedle for delivery of therapeutic agent such as a drug, antibody, orbiologic therapy, a thermal ablator, a cryosurgery probe, acutting/cautery probe, or the like, includes a camera 200 attached at aposition thereon having a known spatial relationship with respect to apoint of interest 502 for the instrument 500. In this example, whereinthe surgical instrument 500 is a needle used to deliver a therapeuticagent, the point of interest 502 is the needle end tip. However, thepoint of interest 502 for surgical instrument 500 need not be limited toneedle tips; the point of interest may also include, depending on thesurgical instrument, the distal end of a scalpel, or the distal portionof the active region of an ablator probe or cryotherapy probe.Localization of the surgical instrument 500 will proceed in accordancewith the techniques described in connection with medical imagingdevices, thus allowing a surgeon to accurately determine, in real-time,the location of point of interest 502 in a known 3D coordinate system.

FIG. 6 depicts an example (a top view and a side view) of a localizablebite block 600. A bite block 600 is a medical tool that is well-known inthe art, particularly with respect to radiotherapy treatments of head orneck lesions/tumors, and is molded to fit a patient's teeth, preferablythe patient's upper teeth. In FIG. 6, the bite block 600 includes acamera 200 attached at a position thereon having a known spatialrelationship with respect to a treatment point on the patient's head orneck. Because the bite block is molded to fit the patient's teeth, whichare a relatively stable reference point, the measurements that are madeto determine the position of any head or neck lesions/tumors relative toa point on the bite block will be relatively constant from session tosession. Through the use of the camera 200 in accordance with theteachings of the present invention, the bite block can also be localizedrelative to a fixed 3D coordinate system, thereby allowing the locationof the lesion/tumor on the head or neck to also be accurately localizedin the fixed 3D coordinate system based on the known relationshipbetween the camera 200 and the lesion/tumor established from a CT scanof the patient with the bite block in place.

In both of the examples of FIGS. 5 and 6, the camera 200 is preferablyattached to the medical tool such that the camera 200 is able to track aremote reference target while that tool is being used in connection witha medical procedure. In other words, during use of the medical tool, thecamera 200 is positioned on the medical tool such that the referencetarget remains in the camera's field of view.

With the localization system of the present invention, and relative toconventional camera-based localization systems, the risk of occlusion isminimized through a greater likelihood of finding a location for thereference target that is within the camera's field of view.

While the present invention has been described above in relation to itspreferred embodiment, various modifications may be made thereto thatstill fall within the invention's scope, as would be recognized by thoseof ordinary skill in the art following the teachings herein. As such,the full scope of the present invention is to be defined solely by theappended claims and their legal equivalents.

1. A method of localizing a medical imaging probe, the method comprising: generating an image of a reference target with a camera that is attached to a medical imaging probe, wherein the reference target is remote from the probe and located in a room at a known position relative to a coordinate system; and determining the position of the probe relative to the coordinate system at least partially on the basis of the generated image of the reference target.
 2. The method of claim 1 wherein the determining step comprises determining, in substantially real-time, the position of the probe relative to the coordinate system at least partially on the basis of the generated image of the reference target.
 3. The method of claim 2 wherein the reference target comprises at least one selected from the group consisting of a plurality of passive reflectors and a plurality of printed marks visible to the camera.
 4. The method of claim 3 wherein the reference target comprises a plurality of printed marks arranged in a grid pattern.
 5. The method of claim 4 wherein the grid pattern is a checkerboard pattern.
 6. The method of claim 1 wherein the medical imaging probe is an ultrasound probe.
 7. The method of claim 6 wherein the ultrasound probe is a freehand ultrasound probe.
 8. The method of claim 6 further comprising: generating at least one image of a patient's region-of-interest (ROI) through use of the ultrasound probe; and spatially registering the at least one generated ROI image relative to the coordinate system at least partially on the basis of the determined probe position.
 9. The method of claim 8 wherein the ROI is a patient's prostate.
 10. A method of localizing a medical tool, the method comprising: generating an image of a reference target with a camera that is attached to a medical tool, wherein the reference target is remote from the medical tool and located in a room at a known position relative to a coordinate system; and determining the position of the medical tool relative to the coordinate system at least partially on the basis of the generated image of the reference target.
 11. The method of claim 10 wherein the reference target comprises a plurality of identifiable marks thereon that are arranged in a known spatial relationship with respect to each other, and wherein the camera is attached to the medical tool at a position thereon having a known spatial relationship with respect to a point of interest on the medical tool.
 12. The method of claim 11 wherein the medical tool is a medical imaging device.
 13. The method of claim 12 wherein the medical imaging device is a freehand ultrasound probe.
 14. The method of claim 11 wherein the medical tool is a surgical instrument.
 15. The method of claim 11 wherein the medical tool is a bite block for use in treating head or neck lesions.
 16. The method of claim 11 wherein the reference target is located in the room at a fixed position, and wherein the determining step comprises determining, in substantially real-time, the position of the medical tool relative to the coordinate system at least partially on the basis of the generated image of the reference target.
 17. A system for localizing a medical imaging device, the system comprising: a reference target having a known position in a fixed coordinate system; a medical imaging device having a field of view and being configured to receive data from which a medical image of a patient within the device's field of view is generated, the medical imaging device being remote from the reference target; a camera attached to the medical imaging device for tracking the reference target and generating at least one image within which the reference target is depicted; and a computer configured to (a) receive the camera image and (b) process the received camera image to determine the position of the medical imaging device's field of view relative to the coordinate system in 3D coordinate space.
 18. The system of claim 17 wherein the position of the medical imaging device's field of view relative to the position of the camera on the medical imaging device is known and wherein the computer is further configured to (a) determine, in substantially real-time, the position of the camera in the coordinate system at least partially on the basis of the received camera image, and (b) determine, in substantially real-time, the position of the medical imaging device's field of view at least partially on the basis of the determined camera position and the known position of the medical imaging device's field of view relative to the position of the camera on the medical imaging device.
 19. The system of claim 17 wherein the reference target comprises a plurality of identifiable marks thereon that are arranged in a known spatial relationship with respect to each other.
 20. The system of claim 19 wherein the reference target comprises a plurality of passive reflectors.
 21. The system of claim 19 wherein the reference target comprises a plurality of printed marks visible to the camera.
 22. The system of claim 21 wherein the printed marks are arranged in a grid pattern.
 23. The system of claim 21 wherein the grid pattern is a checkerboard pattern.
 24. The system of claim 17 wherein the medical imaging device is a freehand ultrasound probe.
 25. The system of claim 24 wherein the camera is attached to the freehand ultrasound probe in close proximity to the probe's field of view.
 26. The system of claim 24 wherein the freehand ultrasound probe is used to generate ultrasound images of an internal organ of a patient.
 27. The system of claim 26 wherein the internal organ is the prostate.
 28. The system of claim 26 wherein the origin of the fixed coordinate system is a machine isocenter of a linear accelerator (LINAC).
 29. The system of claim 28 further comprising a LINAC for targeting a beam of radiation to a tumor on the patient's internal organ, wherein the LINAC includes a gantry, and wherein the reference target is located at a known position relative to the fixed coordinate system on the LINAC gantry.
 30. The system of claim 28 further comprising a LINAC for targeting a beam of radiation to a tumor on the patient's internal organ, wherein the LINAC includes a tray, and wherein the reference target is located at a known position relative to the fixed coordinate system on the LINAC tray.
 31. The system of claim 30 wherein the LINAC tray is a blocking tray.
 32. The system of claim 30 wherein the LINAC tray is a wedge tray.
 33. The system of claim 17 wherein the system comprises a system for localizing a medical imaging device used in connection with a tissue biopsy procedure.
 34. The system of claim 17 wherein the system comprises a system for localizing a medical imaging device used in connection with a spatially localized drug delivery procedure.
 35. The system of claim 17 wherein the system comprises a system for localizing a medical imaging device used in connection with a spatially localized radiotherapy procedure.
 36. The system of claim 17 wherein the system comprises a system for localizing a medical imaging device used in connection with an external beam radiation therapy treatment planning session.
 37. The system of claim 17 wherein the system comprises a system for localizing a medical imaging device used in connection with an external beam radiation therapy treatment delivery session.
 38. A localizable medical imaging probe comprising: a medical imaging probe having a field of view; and a tracking camera attached to the medical imaging probe in a known spatial relationship with respect to the probe's field of view.
 39. The probe of claim 38 wherein the tracking camera is attached to the probe such that the tracking camera images a reference target remote from the probe while the probe is being used to generate images of a patient, the reference target having a known position in a three-dimensional coordinate system.
 40. The probe of claim 39 wherein the medical imaging probe is a freehand ultrasound probe.
 41. The probe of claim 40 wherein the reference target is located at a fixed position in the coordinate system.
 42. The probe of claim 40 wherein the tracking camera is a CCD imager.
 43. The probe of claim 40 wherein the tracking camera is detachable from the probe.
 44. A localizable medical tool comprising: a medical tool for use in a medical procedure with a patient; and a tracking camera attached to the medical tool in a known spatial relationship with respect to a point of interest associated with the medical tool.
 45. The localizable medical tool of claim 44 wherein the tracking camera is attached to the medical tool such that the tracking camera images a reference target remote from the probe while the medical tool is being used in connection with a medical procedure, the reference target having a known position in a three-dimensional coordinate system.
 46. The localizable medical tool of claim 45 wherein the tracking camera is a CCD imager.
 47. The localizable medical tool of claim 45 wherein the medical tool is a medical imaging device.
 48. The localizable medical tool of claim 45 wherein the medical tool is a surgical instrument.
 49. The localizable medical tool of claim 45 wherein the medical tool is a bite block for use in treating head or neck lesions.
 50. A computer readable medium for localizing a medical tool relative to a fixed coordinate system of a room, wherein the medical tool comprises a camera attached thereto at a known position relative to a point of interest on the medical tool, the camera being configured to image a reference target disposed in the room remotely from the medical tool, the reference target having a known position relative to the coordinate system, the computer readable medium comprising: a plurality of executable instructions for processing camera images received from the camera together with known position data to determine the position of the medical tool relative to the coordinate system, wherein the camera images at least partially depict the reference target.
 51. The computer readable medium of claim 50 wherein the medical tool is a medical imaging device.
 52. The computer readable medium of claim 51 wherein the medical imaging device is a freehand ultrasound probe, wherein the reference target comprises a plurality of printed marks arranged in a grid pattern that are visible to the tracking camera, and wherein the coordinate system origin is the machine isocenter of a linear accelerator, and wherein the plurality of executable instructions further comprise a plurality of executable instructions for processing camera images received from the camera together with known position data to determine, in substantially real-time, the position of the medical tool relative to the coordinate system, wherein the camera images at least partially depict the reference target.
 53. The computer readable medium of claim 52 wherein the plurality of executable instructions for processing camera images include executable instructions for applying an edge detection, sharpening and pattern recognition algorithm to the camera images.
 54. A system for localizing a medical tool, the system comprising: a medical tool for use in a medical procedure with a patient; a localization system associated with the medical tool that locates the medical tool in a three-dimensional coordinate system, the localization system comprising a reference target having a fixed and known position in the coordinate system, the reference target being remote from the medical tool; and a computer in communication with the localization system, the computer being programmed to (1) receive data from the localization system, and (2) determine the position of the medical tool in the coordinate system at least partially on the basis of data received from the localization system.
 55. The system of claim 54 wherein the localization system further comprises a sensor attached to the medical tool at a known position with respect to a point of interest on the medical tool, the sensor being configured to sense the reference target and generate data indicative of the reference target's position in the coordinate system, wherein the data received by the computer from the localization system comprises the sensor data from the sensor, and wherein the computer is programmed to determine the position of the medical tool in the coordinate system at least partially on the basis of the sensor data.
 56. The system of claim 55 wherein the sensor is a camera having a field of view within which the reference target at least partially resides, wherein the sensor data comprises image data generated by the camera of at least a portion of the reference target, and wherein the computer is programmed to determine the position of the medical tool in the coordinate system at least partially on the basis of the image data from the camera.
 57. The system of claim 56 wherein the reference target comprises a plurality of identifiable marks thereon disposed in a known spatial relationship with each other.
 58. The system of claim 57 wherein the identifiable marks comprise a plurality of passive reflectors.
 59. The system of claim 58 wherein the identifiable marks comprise a plurality of printed marks visible to the camera.
 60. The system of claim 59 wherein the printed marks are arranged in a grid pattern.
 61. The system of claim 57 wherein the computer is further programmed to determine, in substantially real-time, the position of the medical tool in the coordinate system at least partially on the basis of the image data from the camera.
 62. The system of claim 57 wherein the image data of the reference target generated by the camera comprises image data of at least a portion of the reference target.
 63. The system of claim 58 wherein the medical tool is a medical imaging device.
 64. The system of claim 63 wherein the medical imaging device is a freehand ultrasound probe.
 65. The system of claim 57 wherein the medical tool is a surgical instrument.
 66. The system of claim 57 wherein the medical tool is a bite block for use in treating head or neck lesions.
 67. A system for localizing a medical imaging probe, the system comprising: a reference target having a known position in a fixed coordinate system; a medical imaging probe for receiving data from which a medical image of a patient is generated, the probe being remote from the reference target; a tracking camera attached to the probe for tracking the reference target and generating at least one image within which the reference target is depicted; and a computer configured to (a) receive the camera image and (b) process the received camera image to determine the position of the device relative to the coordinate system in 3D coordinate space. 